(1) Field of the Invention
The present invention relates to a pneumatic tire, particularly to a pneumatic tire having remarkably low rolling resistance and high wet skid resistance without deteriorating wear resistance and other properties.
(2) Description of the Prior Art
Recently, resource saving and energy saving have been socially demanded and investigations for developing automobiles with low gasoline consumption have been directed not only to the improvement of engine but also to the development of so-called low fuel consumption tires by which the power loss is reduced.
It has been generally known that if the rolling resistance of a tire is low, the gasoline consumption of an automobile is saved and the rolling resistance of a tire is greatly controlled by the quality, that is, kind and physical properties of the tread rubber. That is, if materials having low glass transition temperature Tg or low hysteresis loss are used, the rolling resistance can be reduced.
For example, it has been known that a blend of styrene-butadiene copolymer rubber (SBR) with natural rubber (NR) or polybutadiene rubber (BR), or a low styrene SBR is used instead of SBR generally broadly used as a tread rubber. However, these rubbers are extremely lower in the running stability, such as wet skid resistance and braking performance on wet road, and it has been very difficult to concurrently satisfy the rolling resistance and the running stability on wet road at once.
It has been know, as disclosed in Japanese Patent Application Publication No. 13,532/72 and No. 42,792/72, that a rubber composition produced by blending solution-polymerized SBR containing up to 30% by weight of bonded styrene with emulsion-polymerized SBR, BR, synthetic polyisoprene rubber (IR) or the like, and mixing the rubber blend with aromatic process oil, is used in the tire tread. However, the rubber compositions in both of the above described patent applications have improved workability and skid resistance on wet road, but the rubber compositions are still high in the rolling resistance, and are poor in the wear resistance and breaking strength, which are fundamental properties required to rubber for tire.
Recently, it has been known, as disclosed in Japanese Patent Laid Open Application No. 62,248/79, to improve the rolling resistance and running stability by using SBR having a glass transition temperature Tg of not lower than -50.degree. C. and having a styrene content of 20-40% by weight and 1,2-bond content in the bonded butadiene of 50-80% by weight, but all the polymer consists of said SBR, so that satisfactory results have not always been obtained. Further, it has been known, as disclosed in Japanese Patent Laid Open Application No. 50,545/79, to use a rubber blend consisting mainly of a polymer having a Tg of not higher than -50.degree. C. and containing a polymer having a Tg of not lower than the room temperature in the tire tread in order to attain the same objects as described above. However, it has been very difficult to satisfy concurrently the rolling resistance and running stability of a tire while keeping excellent breakage resistance and wear resistance of the tire.
Accordingly, it has never been yet attained to obtain a tire concurrently satisfying both the rolling resistance and running stability while maintaining excellent breakage and wear resistances.
It has been already known from "Proceedings of the Royal Society A274,21 (1963)" that the improvement of the friction coefficient on wet road can be attained by using a material having high hysteresis loss, which dissipates deformation energy of a rubber, because the tread surface contacting a road surface is subjected to deformation at high speed due to fine unevenness of the road surface and therefore the larger is the dissipation of energy due to mechanical loss, the larger the friction force is. But, such a material having the high hysteresis loss is high in the heat generation upon rolling the tire and therefore the rolling resistance is poor.
The inventors have diligently studied the above described drawbacks and noticed, as disclosed in the copending U.S. patent application Ser. No. 231,529, that the deformation at the friction surface of the tread occurs at high speed and the hysteresis loss in the deformation at high speed has a great influence on the friction force, that is, the running stability of the tire, while the deformation of the tread owing to ground contact, which has a great influence on the rolling resistance of the tire, corresponds to the rotation speed of the tire, so that the hysteresis loss at the deformation of less than 100 Hz has a great influence on the rolling resistance. According to temperature-time deducibility by Williams, Landel and Ferry, J. Am. Chem. Soc. 77:3701 (1955) it is supposed that the hysteresis at deformation at high speed controlling the running stability conforms to the hysteresis loss measured at a lower temperature than the temperature at which the tire is used, so that the inventors have studied the conditions under which the hysteresis loss is made to be larger in order to improve the running stability and found that such a hysteresis has a close relation to the hysteresis measured at about 0.degree. C. in the dynamic measurement of 1.5 Hz and concerning the rolling resistance, the hysteresis loss has a close relation to one measured at 50.degree.-70.degree. C. Thus, if the hysteresis loss at about 0.degree. C. is larger and the hysteresis loss at 50.degree.-70.degree. C. is smaller, rubber compositions wherein both the running stability and rolling resistance are concurrently improved, can be obtained.